The GSM (Global System for Mobile Communications) is a pan-European mobile phone system, which is becoming a world-wide standard. FIG. 1 shows the basic structural elements of the GSM system in a simplified manner without detailed description of their properties or the remaining parts of the system. A mobile exchange MSC switches incoming and outgoing calls. Most of its functions are similar to those of the exchange of a public switched telephone network. In addition to these normal functions, the mobile exchange also has functions typical of mobile communications, such as subscriber location management. Mobile stations (MS) are connected to the exchange (MSC) by means of base station systems. A base station system comprises a base station controller (BSC) and base stations (BTS). Each of the base station controllers is used for controlling a plurality of base stations. A BSC performs, for example, handovers in cases where the handover takes place between two base stations controlled by the same BSC.
The GSM system is entirely digital; even speech transmission is carried out entirely digitally. On account of the capacity restrictions of the radio path used, it is necessary to have an encoding method which is different from normal digital encoding methods and which takes into account the special features of speech; such a method is the RPE-LTP method (Regular Pulse Excitation-Long Term Prediction), which utilizes both long and short term prediction. The encoding produces LAR Logarithmic Area Ratio, RPE and LTP parameters for transmission.
In the network, various speech encoding and rate adaption functions have been concentrated in one transcoder unit TRCU (Transcoder/Rate Adaptor Unit). The TRCU may have several alternative locations in the system, depending on the choices made by the manufacturer. When the transcoder TRCU is positioned apart from the base station BTS, information is transmitted between the BTS and the TRCU in so-called TRAU frames of 320 bits, meaning that at a transmission rate of 16 kbit/s it takes 20 ms to transmit one frame. There are four different types of frames, classified by their information content: speech, use/maintenance and data frames, and so-called idle speech frames. To enable synchronization, the first two octets in each frame contain 16 synchronization bits. In addition, the first bit of the 16-bit words (two octets) that form a frame is a synchronization check bit. In addition to data bits containing actual speech, data or use/maintenance information, all frames comprise control bits C1-C21, which transmit information on the type of the frame and a varying amount of other frame type specific information. In both types of the speech frames the last four bits T1-T4 are reserved for timing alignment. FIG. 2 illustrates a speech frame according to GSM Recommendation 08.60, current edition as of Jun. 10, 1993, published by European Telecommunications Standards Institute, Sophia Antipolis, Valbonne, France. The data to be transmitted in the frame consists of speech encoding parameters. First there is a 36-bit group of LAR parameters. Then there are four subframes I, II, III and IV, each of which contains a 56-bit group of LTP-RPE parameters.
The interfaces of the transcoder are a 64 kbit/s PCM (Pulse Code Modulation) interface towards the mobile exchange MSC and a 16 kbit/s GSM interface towards the base station BTS. In connection with these interfaces the GSM recommendations also use the terms uplink and downlink direction: the uplink direction is the direction from the base station unit BTS towards the mobile exchange MSC, and the downlink direction is the reverse direction.
At the beginning of a call a transcoder TRCU is reserved for the call. Synchronization with the TRAU frames from the uplink direction is effected by means of the above-mentioned synchronization bits and synchronization check bits in such a manner that the actual synchronization with the frame takes place by means of the first 16 synchronization bits. The frame synchronization is effected continuously, and the time window used for monitoring is shifted according to possible changes in timing. The subsequent synchronization check bits in the frame are only used for detection of errors occurring within the frame. An error detected in the synchronization check bits does not call for action until in at least three consecutive erroneous TRAU frames are detected, in which case the level of the decoded speech is attenuated until synchronization is restored.
A handover between two base stations BTS can be performed either as an intra-BSC operation within a base station controller BSC using the same PCM line to the exchange or as an inter-BSC operation between base station controllers by changing the PCM line to the exchange. In the existing system, the base station controller BSC performs handovers within a base station controller by switching a new base station in place of the old base station at a random moment in relation to the TRAU frames transmitted by the new base station. This is illustrated in FIG. 3. The column on the left illustrates the TRAU frames transmitted by the old base station BTS1, the column in the middle illustrates the TRAU frames transmitted by the new base station BTS 2, and the column on the right illustrates the TRAU frames received by the transcoder. Base stations BTS1 and BTS2 are not synchronized with each other; the TRAU frames transmitted by them may thus have an arbitrary phase difference .DELTA.t. The transcoder TRCU is synchronized with the frame transmitted by the base station BTS1 at a point of time t.sub.1, when receiving 16 synchronization bits "0" at the beginning of the frame. Thereafter it monitors synchronization bits "1", which regularly occur in the frame, and detects a proper synchronization operation. At a point of time t.sub.2 the base station controller BSC performs a change of base station and switches the transcoder TRCU to a new base station BTS2 while the transcoder is receiving a TRAU frame, whereby the last frame transmitted by the old base station is interrupted. The frame which is transmitted by the new base station BTS2 and whose reception begins at a random point within the frame is attached to the original interrupted frame in the transcoder. The data contained at the end of the last frame whose transmission was started by the old base station BTS1 is thus useless. In the existing systems the transcoder TRCU does not, however, take this into account but decodes the "speech encoding parameters" included in this data. These parameters may have any values, wherefore an undue disturbing sound effect is produced amidst normal speech due to the handover. An error is usually detected in the synchronization check bits, but it does not call for action, as in the following frame, transmitted by the new base station BTS2, the synchronization will be restored.